The present invention relates to a solid electrolyte capacitor using a conducting polymer as a solid electrolyte, and a method of producing the same.
Generally, a solid electrolyte capacitor has a first electrode (anode), a dielectric implemented by an oxide film formed on the first electrode, and a second electrode (cathode) partly implemented by a solid electrolyte formed on the dielectric. The first electrode is constituted by a porous body of valve metal, e.g., tantalum, niobium or aluminum. The solid electrolyte electrically connects the entire dielectric and electrode leads. Therefore, the conductivity of the solid electrolyte should preferably be as high as possible so as to reduce the resistance of the capacitor itself. On the other hand, the solid electrolyte is required to automatically heal electric short-circuiting ascribable to the defects of the dielectric film. For this reason, metal lacking a dielectric healing function cannot be used as a solid electrolyte although it may have a high conductivity. The solid electrolyte has customarily been implemented by, e.g., manganese dioxide or 7, 7', 8, 8'-tetracyanoquinodimethane complex (TCNQ complex) which migrates to an insulator due to heat generated by short-circuit current. Particularly, manganese dioxide resistive to temperature at least above 240.degree. C. is predominant because the solid electrolyte is exposed to temperature between 240.degree. C. and 260.degree. C. when the capacitor is mounted to a printed circuit board.
As stated above, a substance to implement the solid electrolyte of a solid electrolyte capacitor must satisfy a condition that it has a high conductivity, a condition that it has a dielectric repairing function, and a condition that it is resistive to healing above 240.degree. C.
While manganese dioxide customarily used as a solid electrolyte is satisfactory as to the dielectric healing function and heat resistance, its conductivity (about 0.1 S/cm) is not sufficient. In light of this, electrolyte capacitors using polypyrrole, polythiophene, polyaniline and other conductive high polymers satisfying the above three conditions as a solid electrolyte are under development. Capacitors using polypyrrole have already been put on the market.
Generally, a capacitor using a conducting polymer has four different problems to be solved, as follows. First, the conducting polymer must be formed over the entire surface of the inside of a porous body. Second, the conductivity of the polymer must not decrease in a high temperature atmosphere to which the capacitor is exposed. Third, a conducting polymer layer must be formed on an oxide film with a thickness great enough to withstand stresses ascribable to the expansion and contraction of a seal resin. Fourth, The conducting polymer layer must be easy to form for reducing the production cost of the capacitor.
To solve the above problems, polymerizing a thiophene derivative with a ferric compound has been proposed in order to use the resulting polymer as a solid electrolyte, as taught in, e.g., Japanese Patent Laid-Open Publication No. 2-15611 and U.S. Pat. No. 4,910,645. The above polymer is resistive to heat more than the polymer of a pyrrole derivative and therefore desirable to solve the second problem.
Japanese Patent Application No. 8-185831 discloses a solid electrolyte capacitor including a solid electrolyte layer implemented by a polymer layer doped with an organic sulfonic acid having a large molecule size, and a method of producing a solid state capacitor which forms a polymer layer by use of a ferric compound, a silver compound or similar compound as an oxidizer.
However, the method taught in the above Laid-Open Publication No. 2-15611 has some problems left unsolved, as follows. In a 125.degree. C., 150.degree. C. or similar high temperature atmosphere, conductivity decreases due to dedoping because the amount of dopant is locally short relative to a conducting polymer produced by an oxidizer. Further, leak current and ESR (Equivalent Serial Resistance) noticeably increase during solder heat test and thermal shock test. An increase in leak current stems from the fact that the thickness of the conducting polymer layer is locally short and cannot absorb mechanical stresses, causing the stresses to directly act on a capacitor device. An increase in ESR is ascribable to the fact that separation occurs within the conducting polymer layer due to mechanical stresses.
Technologies relating to the present invention are also taught in, e.g., Japanese Patent Laid-Open Publication No. 8-45790.